The worm-geared condensing tramway locomotive of Mr.
W. Wilkinson combines a few original elements. Acting on the
results of Mr. Dewrance's experiments on the evaporative activity
of different parts of a locomotive boiler, Mr. Wilkinson designed a
short locomotive boiler, having flue tubes only 12 inches in
length. His object was to maintain the water level in the boiler
practically constant whether on level ground or on steep incline
such as 1 in 11. The tubes are not straight, but of an ogee form,
in order to yield to expansion without the liability of breakage.
The steam is exhausted into a condenser fitted with tubes also of
ogee form presenting between 200 and 300 square feet of condensing
surface. The steam surrounds the tubes, and the air for cooling
them is drawn through them into the ash-pan to support combustion.
The steam that remains uncondensed passes into a superheater in the
smoke-box -- a rectangular box fixed against the tube plate,
traversed by tubes coincident with those of the boiler, through
which the burnt gases flow into the smoke box.
While running on a level line, with a heavy load, in
or nearly in mid-gear, there is scarcely sufficient steam left
uncondensed to cause the necessary draught, and in such a case the
draught is sharpened by reducing the blast orifice, in applying a
thimble to the top of the blast pipe, which is done by hand with a
lever.
In the fire-box a number of Field tubes are fixed to
the root between the roof-stays.
The power of the engine is transmitted through
worm-gear to the driving axle. The worm is double-threaded, and one
turn of the axle is made for 7½ turns of the worm. The
diameters of the worm and the wheel are equal, or nearly so-- about
12 inches in diameter ; and the obliquity of the worm-thread is so
considerable that, on a falling gradient of 1 in 70, the worm is
propelled by the force of gravity. The worm is of Siemens steel,
and the toothed wheel runs in an oil bath, providing constant
lubrication. the carrying wheels are 3 feet 2 inches in diameter.
There are two cylinders working to a logitudinal crank axle. It is
stated that the engine can draw a loaded car, weighing 3 tons, full
of passengers, on a level or nearly level road, with the valves in
mid gear ; the steam being cur off at 9 per cent. of the stroke. At
the regulation speed of 8 miles per hour, the speed of the pistons
is 436 feet per minute. The engine has been worked at a speed of 16
miles per hour. The machinery is compactly placed, and the
cylinders, guide bars, valve gear, foundation plate, worm, and
plummer-blocks can be removed for repair by undoing six bolts, and
a duplicate substituted.
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Wilkinson.